APRIL is expressed as a type-II transmembrane protein, but unlike most other TNF family members it is mainly processed as a secreted protein and cleaved in the Golgi apparatus where it is cleaved by a furin convertase to release a soluble active form (Lopez-Fraga et al., 2001, EMBO Rep 2:945-51). APRIL assembles as a non-covalently linked homo-trimer with similar structural homology in protein fold to a number of other TNF family ligands (Wallweber et al., 2004, Mol Biol 343, 283-90). APRIL binds two TNF receptors: B cell maturation antigen (BCMA) and transmembrane activator and calcium modulator and cyclophilin ligand interactor (TACI) (reviewed in Kimberley et al., 2009, J Cell Physiol. 218(1):1-8). In addition, APRIL has recently been shown to bind heparan sulphate proteoglycans (HSPGs) (Hendriks et al., 2005, Cell Death Differ 12, 637-48). APRIL has been shown to have a role in B cell signalling and drive both proliferation and survival of human and murine B cells in-vitro (reviewed in Kimberley et al., 2009, J Cell Physiol. 218(1):1-8).
APRIL is predominantly expressed by immune cell subsets such as monocytes, macrophages, dendritic cells, neutrophils, B-cells, and T-cells, many of which also express BAFF. In addition, APRIL can be expressed by non-immune cells such as osteoclasts, epithelial cells and a variety of tumour tissues (reviewed in Kimberley et al., 2009, J Cell Physiol. 218(1):1-8). In fact, APRIL was originally identified based on its expression in cancer cells (Hahne et al., 1998, J Exp Med 188, 1185-90). High expression levels of APRIL mRNA were found in a panel of tumour cell lines as well as human primary tumours such as colon, and a lymphoid carcinoma.
A retrospective study under 95 Chronic Lymphocytic Leukaemia (CLL) CLL patients showed increased levels of APRIL in serum, which correlated with disease progression and overall patient survival, with a poorer prognosis for patients with high APRIL serum levels (Planelles et al., 2007, Haematologica 92, 1284-5). Similarly, (increased levels of) APRIL was shown to be expressed in Hodgkin's lymphoma, Non-Hodgkin's lymphoma (NHL) and Multiple Myeloma (MM) (reviewed in Kimberley et al., 2009, J Cell Physiol. 218(1):1-8). A retrospective study in DLBCL patients (NHL) showed that high APRIL expression in cancer lesions correlated with a poor survival rate (Schwaller et al., 2007, Blood 109, 331-8). Recently, APRIL serum levels in serum from patients suffering from colorectal cancer were shown to have a positive diagnostic value (Ding et al., 2013, Clin. Biochemistry, http://dx.doi.org/10.1016/j. clinbiochem.2013.06.008).
Due to its role in B cell biology APRIL also plays a role in many autoimmune diseases. Increased serum levels of APRIL have been reported in many SLE patients (Koyama et al., 2005, Ann Rheum Dis 64, 1065-7). A retrospective analysis revealed that APRIL serum levels tended to correlate with anti-dsDNA antibody titres. Also in the synovial fluid of patients with inflammatory arthritis significantly increased APRIL levels as compared with those with patients suffering from non-inflammatory arthritis such as osteoarthritis were detected (Stohl et al., 2006, Endocr Metab Immune Disord Drug Targets 6, 351-8; Tan et al., 2003, Arthritis Rheum 48, 982-92).
Several studies focused on the presence of APRIL in the sera of patients suffering from a wider range of systemic immune-based rheumatic diseases (now also including Sjögren's syndrome, Reiter's syndrome, psoriatic arthritis, polymyositis, and ankylosing spondylitis) and found significantly increased APRIL levels in these patients, suggesting an important role for APRIL in these diseases as well (Jonsson et al., 1986, Scand J Rheumatol Suppl 61, 166-9; Roschke et al., 2002, J Immunol 169, 4314-21). In addition, increased APRIL serum levels were detected in serum from patients suffering atopic dermatitis (Matsushita et al., 2007, Exp. Dermatology 17, 197-202). Also, serum APRIL levels are elevated in sepsis and predict mortality in critically ill patients (Roderburg et al., J. Critical Care, 2013, http://dx.doi.org/10.1016/j.jcrc.2012.11.007). Finally, increased APRIL expression has also been linked to Multiple Sclerosis (MS). APRIL expression was found to be increased in the astrocytes of MS sufferers compared with normal controls. This is in line with the described APRIL expression in glioblastomas and in the serum of glioblastoma patients (Deshayes et al., 2004, Oncogene 23, 3005-12; Roth et al., 2001, Cell Death Differ 8, 403-10).